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Rewrite the textbooks: A new type of cell membrane

Any biology textbook will tell you that all cells—human, bacteria, plant, what have you—build their membranes using phospholipids, fatty molecules that contain phosphorus. But a new study published online in Nature suggests that phytoplankton, the plant-like microorganisms living on the surface of oceans, may be in a league all of their own; unlike other organisms, they don't necessarily make their membranes with phospholipids.

"Phosphorus is an essential nutrient, but in some parts of the ocean that nutrient is very scarce," says lead study author Benjamin Van Mooy, a geochemist at the Woods Hole Oceanographic Institution in Woods Hole, Mass. "The phytoplankton that live there are doing just fine. We have found that the reason for this is that they have the ability to make their membranes without using phosphorous."

In addition to correcting a long-held misconception, this study may provide insight into how phytoplankton deal with the stresses of global warming, he says, noting that around one third of the carbon dioxide (CO2) released by burning fossil fuels accumulates in the ocean and may be absorbed by phytoplankton.

"If oceans become warmer, some are thinking that oceans may become more starved of phosphorus," Van Mooy says. "This is definitely a mechanism for plankton to continue to thrive [and continue soaking up carbon dioxide] in the absence of phosphorus."

Long before Van Mooy and his colleagues published this research, scientists knew that phytoplankton thrive in ocean environments such as the North Pacific and the Mediterranean Sea, where phosphorus supplies are scarce; they just didn't know how the phytoplankton pulled it off. Some had hypothesized that, instead of building membranes with phospholipids, they were using "substitute lipids," or fatty molecules that don't contain phosphorus but other atoms such as sulfur.

Laboratory studies had actually revealed that at least two species of phytoplankton (among the thousands that exist) did have this special ability, but scientists had not shown that the phenom occurs in nature, Van Mooy says.

In April 2007, Van Mooy and his colleagues traveled by boat to the Sargasso Sea, a large tract of Atlantic Ocean stretching from the Eastern seaboard of the U.S. and the Caribbean to Europe and North Africa. The Sargasso is known to have very low levels of phosphorus. The researchers took samples of plankton-rich water, filtered out the plankton, and brought it back to their lab [see photo].

They discovered that the plankton cell membranes contained about four times more substitute lipids than phospholipids. These substitute lipids are the norm rather than the exception in the world's oceans, most of which have scanty supplies of phosphorus, Van Mooy says: "70 percent of the world is covered in water; in those areas, substitute lipids are dominant."

Up next: a study of lakes to determine if freshwater plankton also use substitute lipids to build their cell membranes.

Photo: Woods Hole Oceanographic Institution’s Benjamin Van Mooy (right) and Justin Ossolinski (left) tend an on-deck incubator aboard the /R/V Oceanus/ during a recent cruise to the Gulf Stream in search of lipid substitutions by phytoplankton (photo credit: Kimberly Popendorf)

 

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